I have a jealousy problem, help?

My fiancee has a best friend that he's known like all his life. And he loves her and w/e cause she's his best friend. But I guess after a couple of years they like never talk. But he still cares for her. Every now and again @ like the worst times she'll pop up. And it bothers me cause she's not there for him like I am :'( All she ever does is vent to him. And I still let him talk to her cause I mean its his best friend... It makes me very uncomfortable... And like it upsets him that I'm uncomfortable. How does he expect me to feel? He loves her :'( he told me he'd never leave me for her...but still don't I have a right to be upset. And like I have a best friend too, but I don't love him. And we pretty much have same relationship as my fiancee nd his best friend. So he 'claims' he doesn't care about my guy friend but I know he does. So he shouldn't judge me for being upset... I love him very much, please tell me what to do.. I don't want to be like this. I just...I'm uncomfortable.....